Bus oriented bi-directional video interconnection systems are known and used, for example, for interconnecting component video apparatus such as video tape recorders, video disc players, television tuners, video cameras, video monitors and so forth. Such applications require the ability of the driver to apply video signals to the bus with a relatively low source or sink impedance and to isolate video signals from the bus with a relatively high "off state" impedance. Such a characteristic is called a "tri-state" operation because of the three possible drive conditions of (1) supplying or "sourcing" current to the bus with a low impedance, (2) removing or "sinking" current from the bus with a low impedance and (3) isolating or de-coupling the video signal from the bus with a very high impedance, essentially an open circuit.
One may achieve such tri-state operation by coupling the output of a video driver to a bus by means of a mechanical relay. Mechanical relays have excellent electrical characteristics for this purpose in terms of on and off impedances but are relatively expensive and may be relatively bulky, unreliable and slow. Solid state switches, such as transmission gates, may be used for selectively coupling video signals to a bus but typically exhibit a relatively high "on" resistance and the "on" resistance may also vary with signal levels thus introducing undesirable signal distortions.
One solution to the problem of providing tri-state operation which overcomes the problem of poor "on-state" characteristics of solid state switches is described by Beyers, Jr. in U.S. Pat. No. 4,581,645 entitled DISTRIBUTED SWITCHED COMPONENT AUDIO/VIDEO SYSTEM which issued Apr. 8, 1986. In an exemplary driver amplifier therein described, video coupling to a bus is provided by means of a PIN diode. As explained by Beyers, Jr., such diodes have excellent isolation characteristics at video frequencies but have relatively poor "on-state" impedance characteristics (e.g., a relatively high and variable impedance when biased "on"). Beyers solved the problem for PIN diode switches by incorporating the PIN switching diode in a negative feedback path with the driver amplifier. The negative feedback is applied when the diode is biased "on" and effectively reduces and stabilizes the on-state diode resistance when coupling video signals to the bus. Provisions are made for applying reverse DC bias to the diode when isolating the video signals from the bus. However, the use of special switching diodes (PIN diodes) and the application of negative feedback to the diode results in a relatively complex switching circuit.